An exemplary configuration of a first technology for suppressing the echo caused by acoustic system coupling between a sound pickup apparatus and a loudspeaker in the hands-free telephone set is shown in FIG. 30. In FIG. 30, a sound signal (which is called a far-end signal) of a call partner being applied to an input terminal 10 is produced as a far-end sound from a loudspeaker 2. The necessary sound, for example, speaker's voice (which is called near-end sound) enters a microphone 1, and simultaneously therewith, the far-end sound produced from the loudspeaker 2 leaks into it through space. This crosstalk is called an echo. Further, the acoustic transmission system including the loudspeaker 2 and the microphone 1, which handles sound-related signals ranging from the far-end signal to the output signal of the microphone 1, is called an echo path.
Only the near-end sound is desired to be sent as a near-end signal from an output terminal 9, and the echo of the far-end sound that has been produced from the loudspeaker 2 and yet has leaked into the microphone 1 is desired to be removed. When the far-end sound leaks at a high volume level, the delayed far-end sound is audible as an echo to a call partner, so that it becomes difficult to originate a call in the far-end side. The technique, which, as a rule, is applied so as to address this problem, is a technique that employs a linear echo canceller. The linear echo canceller is described, for example, in Non-patent document 1.
In FIG. 30, a linear echo canceller 3 simulates a transfer function of the echo path. The linear echo canceller 3 uses the signal inputted into the loudspeaker 2 (far-end signal) to generate a simulated signal of the echo, which leaks into the sound received by the microphone 1, namely, an echo replica signal by employing this simulated transfer function, and a subtracter 4 subtracts the echo replica signal from the sound received by the microphone 1, thereby to generate the near-end signal. Additionally, a sound detector 5 inputs the output of the microphone 1, the output of the linear echo canceller 3, the output of the subtracter 4, and the far-end signal, respectively, detects whether the near-end sound exists, outputs 0 or a very small value as a sound detection result when the near-end sound exists, and outputs a large value when no near-end sound exists.
An operation of the linear echo canceller 3 will be explained by employing FIG. 31. FIG. 31 is a view illustrating an exemplary configuration of the linear echo canceller 3. An exemplary configuration of the linear echo canceller 3 shown in FIG. 31 is configured of an adaptive filter 30 and a multiplier 35. The adaptive filter 30 has the far-end signal inputted from a terminal 31 as an input, and outputs a result by a linear filter operation from a terminal 32. Herein, a filter coefficient that is used for performing the linear filter operation in the adaptive filter 30 is updated every moment. The above update is performed by employing a correlative arithmetic operation so that the subtraction result received from the terminal 33 is minimized. As a result, the subtraction result that is added to the terminal 33 includes the minimized component correlative to the far-end signal. That is, the echo of the far-end signal results in being removed.
The multiplier 35 is inserted into the path from the terminal 33 to the adaptive filter 30 so as to control an amount of the update of the filter coefficient in the adaptive filter 30. In the case that the multiplier 35 does not exist, when the filter coefficient of the adaptive filter 30 is updated in a situation in which the near-end sound exists, the filter coefficient is disordered temporarily, and the removal amount of the echo is diminished. The multiplier 35 sends to the adaptive filter 30 a result obtained by multiplying the subtraction result received from the terminal 33 by the sound detection result coming from the sound detector 5, which has been received from a terminal 34. The update of the filter coefficient of the adaptive filter 30 is suppressed and the disorder of the filter coefficient disappears because the sound detection result becomes 0 or a very small value when the near-end sound exists. As a result, a high echo removal amount is gained.
In such a manner, the linear echo canceller can remove the echo of the far-end signal by employing the adaptive filter. Various configurations such as an FIR type, an IIR type, and a lattice type can be employed for the adaptive filter.
A second technology for suppressing the echo caused by the acoustic-system coupling between the sound pickup apparatus and the loudspeaker is described in Patent document 1. This second technology modifies the echo replica signal of the echo canceller based upon a rotary angle of a hinge of a folding-type mobile telephone apparatus. Specifically, the second technology includes a control signal generation means for detecting the rotary angle of the hinge and outputting a control signal equivalent to the above rotary angle, and a sound control means having an echo control means for suppressing the echo in accordance with the foregoing control signal, and the foregoing echo suppression means includes a coefficient selection circuit for storing a plurality of predetermined echo path tracking coefficients, which are used to generate a pseudo echo that tracks a fluctuating echo path, and using the foregoing control signal as an address designation signal to output the foregoing echo path tracking coefficient, an adaptive control circuit for outputting an pseudo echo modification signal, which is used to modify the foregoing pseudo echo, based upon the foregoing echo path tracking coefficient, a pseudo echo generation circuit for generating the pseudo echo based upon the foregoing pseudo echo modification signal, and a subtraction circuit for subtracting the foregoing pseudo echo from the echo inputted from a sound inputting means.
A third technology for suppressing the echo caused by the acoustic-system coupling between the sound pickup apparatus and the loudspeaker is described in Patent document 2. When either the output signal of the sound pickup apparatus or the signal obtained by subtracting the output signal of the echo canceller from the output signal of the sound pickup apparatus is defined as a first signal and the foregoing output signal of the echo canceller is defined as a second signal, the third technology calculates an estimation value of a degree of the crosstalk of the echo from the foregoing first signal and the foregoing second signal, and corrects the foregoing first signal based upon this calculated estimation value. As the foregoing estimation value, a ratio of an amount corresponding to an amplitude or a power of the foregoing first signal to an amount corresponding to that of the foregoing second signal during a time period in which no near-end sound is detected and employed. Further, preferably, the third technology calculates an estimation value of a degree of the crosstalk of the echo from the foregoing first signal and the foregoing second signal for each frequency component of the foregoing first and second signals, and corrects the foregoing first signal based upon this calculated estimation value.
Further, in a fourth technology shown in Patent document 3, a constant based upon pre-measurement is employed as a coefficient of a degree of the crosstalk of the echo in the Patent document 2.    Patent document 1: JP-P1996-9005A    Patent document 2: JP-P2004-056453A    Patent document 3: International Laid-Open Publication WO2007/049643A1    Non-patent document 1: The paper “The hands-free telephone problem: an annotated bibliography update” by Eberhard Hansler, Annals of Telecommunications, 1994, pages 360 to 367.